The present invention relates to medical devices. More particularly, the invention relates to a removable filter that can be percutaneously placed in and removed from a blood vessel, such as the vena cava, of a patient.
A need for filtering devices arises in trauma patients, orthopedic surgery patients, neurosurgery patients, or in patients having medical conditions requiring bed rest or non-movement. During such medical conditions, the need for filtering devices arises due to the likelihood of thrombosis in the peripheral vasculature of patients wherein thrombi break away from the vessel wall, risking downstream embolism or embolization. For example, depending on the size, such thrombi pose a serious risk of pulmonary embolism wherein blood clots migrate from the peripheral vasculature through the heart and into the lungs.
A filtering device can be deployed in the vena cava of a patient when, for example, anticoagulant therapy is contraindicated or has failed. Typically, filtering devices are permanent implants, each of which remains implanted in the patient for life, even though the condition or medical problem that required the device has passed. In more recent years, filters have been used or considered in preoperative patients and in patients predisposed to thrombosis which places the patient at risk for pulmonary embolism.
The benefits of a vena cava filter have been well established, but improvements may be made. For example, filters generally have not been considered removable from a patient due to the likelihood of endotheliosis of the filter during treatment. After deployment of a filter in a patient, proliferating intimal cells begin to accumulate around the filter struts which contact the wall of the vessel. After a length of time, such ingrowth prevents removal of the filter without risk of trauma, requiring the filter to remain in the patient. As a result, there has been a need for an effective filter that can be removed after the underlying medical condition has passed.
In addition, further improvements may be made related to the delivery or retrieval of vena cava filters. It is desirable to lessen the scratching or scraping of the anchoring hooks against outer walls of an tube or a blood vessel while maintaining the effectiveness of the filter. For example, an anchoring feature should prevent migration toward the heart while allowing easy, non-traumatic removal when the patient's medical condition no longer exists. A vena cava filter can be subjected to considerable forces when the filter is substantially full of clot and the patient strains or performs a valsalva. This tends to dilate the vena cava and force a large volume of blood toward the heart. There have been incidences where filters designed for permanent implantation have been dislodged and migrated into the heart when confronted with such a challenge.
Thus, the anchoring hooks typically maintain the filter in place by penetrating the vessel wall. However, anchoring hooks also may scrape or scratch the blood vessel during delivery or retrieval.